In domestic heating arrangements utilising an accumulator means for temporary storage of a heated fluid, such as water, some form of heating arrangement is used for heating the fluid in the accumulator means. For instance, when using a water heating boiler for heating the water in an accumulator tank it is desirable to heat the water in the boiler to a predetermined temperature before charging the accumulator tank with hot water. Once the water circulating in the boiler has reached a predetermined temperature, a thermostatic mixing valve connecting the boiler and the accumulator tank is actuated to initiate a relatively slow charging of the accumulator tank. A slow charging is preferred in order to achieve stratification of the hot and cold water in the accumulator tank. By avoiding mixing of the hot water entering an upper portion of the tank with the cold water drawn from a lower portion of the tank, the hot and cold water will be separated by a boundary layer in a horizontal plane as the tank is gradually filled with hot water.
An example of a thermostatic mixing valve suitable for this purpose is “Laddomat 21”® available from Termoventiler AB. This thermostatic mixing valve will allow water to circulate in the boiler until a predetermined operating temperature is achieved. The valve will then open partially in response to a heat sensitive body in the valve and begin drawing a limited flow cold water from the accumulator tank into the boiler, while a similar flow of heated water from the boiler is supplied to the accumulator tank. This thermostatic mixing valve will operate satisfactorily in domestic heating arrangements with a rated power output up to about 80 kW.
However, the known valve has proven less suitable for smaller heating arrangements with a rated output up to 30 kW. In such relatively small plants the relatively low fluid flow rate and, in some cases, relatively large pressure differences in the fluid system the known valve may experience control problems and cause flow fluctuations.
Hence, a problem with prior art solutions is to achieve a thermostatic mixing valve providing a stable control of the mixing of two fluid flows at relatively low flow rates and/or at relatively large pressure differences in the system.